Medical procedures often require locating and treating areas within a patient's body. Imaging systems, including x-ray, MRI, CT, and ultrasound have been used to help locate areas or particular targets within the body. While the imaging systems can be very useful in some situations, they can be limited to two dimensional information and may be unusable or difficult to use in certain procedures to provide real time three dimensional location information about a target.
Many noninvasive medical procedures, such as radiation therapy and surgical procedures, require precise location information about the target to minimize the extent of collateral damage to healthy tissue around the target. Markers have been used to locate targets on and in a patient's body in preparation for a medical procedure. One example includes the use of gold fiducials, which are solid, inert, metal beads that can be implanted in a patient at or near a tumor or other target that may be difficult to accurately detect using conventional imaging systems. The fiducial markers are passive markers that are easy to detect with imaging systems such as x-ray or ultrasound systems, but the passive markers do not provide active real-time location information during a medical procedure.
Active, implantable marker assemblies that generate a detectable signal have been used to locate a selected target or the like in real time. Many of the active markers are implantable in a patient, but they are hard-wired to a power source or other equipment external from the patient. These hard-wired markers are removed from the patient's body after a procedure or a series of procedures are concluded. The hard-wired markers are often fairly large in order to provide desired signal strength, clarity, or other performance characteristics needed during the procedure. The patient's body can typically temporarily handle the larger marker during the medical procedure before the marker must be removed. The removal process, however, requires an additional invasive procedure to the patient's body.
Leadless active markers also referred to as “wireless” active markers, have been developed to be implanted in a patient's body at or near a selected target, such as a tumor. The wireless active markers are typically activated or energized to generate a detectable signal used to locate the marker in the patient's body. Some wireless markers contain a power source, such as a battery, that provides the power to generate a signal detectable from outside the patient's body. The battery-powered markers, however, typically must be removed after the medical procedure because the caustic materials in the battery are not suitable to leave in a patient.
The conventional, wireless active markers are also often fairly large in order to provide a range of operating characteristics that allow the marker to be accurately located within the patient's body. Wireless active markers have experienced a trade-off between physical size and signal strength. Larger active markers have been needed to provide the required signal strength for detection and must be tuned adequately enough so that the detection system can detect the marker's signal. The large active markers, however, have drawbacks, including a reduced accuracy of determining the marker's precise location relative to a target, the degree of invasiveness needed to implant the markers in the body, and the costs of producing accurately tuned markers.